electromagnetism In physics, electromagnetism is an interaction that occurs between particles with electric charge. It is the second-strongest of the four fundamental interactions, after the strong force, and it is the dominant force in the interactions o ...

, a helicon is a low-frequency

electromagnetic wave In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, infrared, (visible) ...

that can exist in bounded plasmas in the presence of a

magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...

. The first helicons observed were atmospheric whistlers, but they also exist in solid conductorsBowers, R., Legéndy, C. R., and Rose, F. E. (November 1961) "Oscillatory galvanomagnetic effect in metallic Sodium". Physical Review Letters 7 (9): 339–341. DOI: 10.1103/PhysRevLett.7.339. or any other electromagnetic plasma. The

electric field An electric field (sometimes E-field) is the physical field that surrounds electrically charged particles and exerts force on all other charged particles in the field, either attracting or repelling them. It also refers to the physical field ...

in the waves is dominated by the

Hall effect The Hall effect is the production of a voltage difference (the Hall voltage) across an electrical conductor that is transverse to an electric current in the conductor and to an applied magnetic field perpendicular to the current. It was dis ...

, and is nearly at right angles to the

electric current An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is measured as the net rate of flow of electric charge through a surface or into a control volume. The movi ...

(rather than parallel as it would be without the magnetic field); so that the propagating component of the waves is corkscrew-shaped (

helical Helical may refer to: * Helix, the mathematical concept for the shape * Helical engine, a proposed spacecraft propulsion drive * Helical spring, a coilspring * Helical plc, a British property company, once a maker of steel bar stock * Helicoil A t ...

) – hence the term “helicon,” coined by Aigrain. Helicons have the special ability to propagate through pure

metal A metal (from ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, e ...

s, given conditions of low temperature and high magnetic fields. Most electromagnetic waves in a normal conductor are not able to do this, since the high conductivity of metals (due to their free electrons) acts to screen out the electromagnetic field. Indeed, normally an electromagnetic wave would experience a very thin

skin depth Skin effect is the tendency of an alternating electric current (AC) to become distributed within a conductor such that the current density is largest near the surface of the conductor and decreases exponentially with greater depths in the co ...

in a metal: the electric or magnetic fields are quickly reflected upon trying to enter the metal. (Hence the shine of metals.) However, skin depth depends on an inverse proportionality to the square root of

angular frequency In physics, angular frequency "''ω''" (also referred to by the terms angular speed, circular frequency, orbital frequency, radian frequency, and pulsatance) is a scalar measure of rotation rate. It refers to the angular displacement per unit ti ...

. Thus a low-frequency electromagnetic wave may be able to overcome the skin depth problem, and thereby propagate throughout the material. One property of the helicon waves (readily demonstrated by a rudimentary calculation, using only the Hall effect terms and a

resistivity Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that measures how strongly it resists electric current. A low resistivity indicates a material that readily allows ...

term) is that at places where the sample surface runs parallel to the magnetic field, one of the modes contains electric currents that “go to infinity" in the limit of perfect conductivity; so that the

Joule heating Joule heating, also known as resistive, resistance, or Ohmic heating, is the process by which the passage of an electric current through a conductor produces heat. Joule's first law (also just Joule's law), also known in countries of former US ...

loss in such surface regions tends to a non-zero limit.Legéndy, C. R. (September 1964) "Macroscopic theory of helicons". The Physical Review 135 (6A): A1713–A1724. DOI:10.1103/PhysRev.135.A1713. The surface mode is especially prevalent in cylindrical samples parallel to the magnetic field, a configuration for which an exact solution has been found for the equations, and which figures importantly in subsequent experiments. The practical significance of the surface mode, and its ultra-high current density, was not recognized in the original papers, but came to prominence a few years later when BoswellBoswell, R. W. (October 1984) "Very efficient plasma generation by whistler waves near the lower hybrid frequency". Plasma Physics and Controlled Fusion 26 (10): 1147–1162. DOI:10.1088/0741-3335/26/10/001. discovered the superior plasma generating ability of helicons – achieving plasma charge densities 10 times higher than had been achieved with earlier methods, without a magnetic field.Boswell, R. W. and Chen F. F. (December 1997) "Helicons – the early years". IEEE Transactions on Plasma Science 25 (6): 1229–1244. DOI: 10.1109/27.650898. Since then, helicons found use in a variety of scientific and industrial applications – wherever highly efficient plasma generation was required,Chen, F. F. (December 1996) "Helicon plasma sources" in: High Density Plasma Sources: Design, Physics and Performance, Oleg A. Popov (ed) (Elsevier-Noyes) print , ebook . as in

nuclear fusion Nuclear fusion is a reaction in which two or more atomic nuclei are combined to form one or more different atomic nuclei and subatomic particles ( neutrons or protons). The difference in mass between the reactants and products is manife ...

reactorsMarini, C., Agnello, R., Duval, B. P., Furno, I., Howling, A. A., Jacquier, R., Karpushov, A. N., Plyushchev, P., Verhaegh, K., Guittienne, Ph., Fantz, U., Wünderlich, D., Béchu, S., and Simonin, A. (January 2017) "Spectroscopic characterization of H₂ and D₂ helicon plasmas generated by a resonant antenna for neutral beam applications in fusion." Nuclear Fusion 57:036024 (9pp) DOI:10.1088/1741-4326/aa53eb and in

space propulsion Spacecraft propulsion is any method used to accelerate spacecraft and artificial satellites. In-space propulsion exclusively deals with propulsion systems used in the vacuum of space and should not be confused with space launch or atmospheric ...

(where the helicon double-layer thrusterCharles, C., Boswell, R. W., and Lieberman, M. A. (December 2006) "Xenon ion beam characterization in a helicon double layer thruster." Applied Physics Letters 89:261503 (3 pgs) DOI: 10.1063/1.2426881. and the

Variable Specific Impulse Magnetoplasma Rocket The Variable Specific Impulse Magnetoplasma Rocket (VASIMR) is an electrothermal thruster under development for possible use in spacecraft propulsion. It uses radio waves to ionize and heat an inert propellant, forming a plasma, then a magnetic ...

Longmier, B. W., Squire, J. P., Cassady, L. D., Ballenger, M. G. Carter, M. D., Olsen, C., Ilin, A. V., Glover, T. W., McCaskill, G. E., Chang Diaz, F. R., Bering III, E. A., and Del Valle, J. (September 2011) “VASIMR® VX-200 Performance Measurements and Helicon Throttle Tables Using Argon and Krypton.” 32nd International Electric Propulsion Conference, held in Wiesbaden, Germany, September 11–15, 2011 (Wiesbaden: IEPC-2011-156). both make use of helicons in their plasma heating phase). Helicons are also utilized in the procedure of

plasma etching Plasma etching is a form of plasma processing used to fabricate integrated circuits. It involves a high-speed stream of glow discharge ( plasma) of an appropriate gas mixture being shot (in pulses) at a sample. The plasma source, known as etch spec ...

,Boswell, R. W. and Henry D. (15 November 1985) "Pulsed high rate plasma etching with variable Si/SiO₂ selectivity and variable Si etch profiles". Applied Physics Letters 47 (10): 1095–1097 DOI: 10.1063/1.96340. used in the manufacture of computer microcircuits.Poulsen, R. G. (1977) "Plasma etching in integrated circuit manufacture – A review" Journal of Vacuum Science and Technology 14 (1): 266 DOI: 10.1116/1.569137 A helicon discharge is an excitation of plasma by helicon waves induced through radio frequency heating. The difference between a helicon plasma source and an

inductively coupled plasma An inductively coupled plasma (ICP) or transformer coupled plasma (TCP) is a type of plasma source in which the energy is supplied by electric currents which are produced by electromagnetic induction, that is, by time-varying magnetic fields. Ope ...

(ICP) is the presence of a magnetic field directed along the axis of the antenna. The presence of this magnetic field creates a helicon mode of operation with higher ionization efficiency and greater electron density than a typical ICP. The Australian National University, in Canberra, Australia, is currently researching applications for this technology. A commercially developed magnetoplasmadynamic engine called

VASIMR The Variable Specific Impulse Magnetoplasma Rocket (VASIMR) is an electrothermal thruster under development for possible use in spacecraft propulsion. It uses radio waves to ionize and heat an inert propellant, forming a plasma, then a magnetic ...

also uses helicon discharge for generation of plasma in its engine. Potentially, helicon double-layer thruster plasma-based rockets are suitable for interplanetary travel.

References

{{Reflist Electromagnetic radiation

See also

References